Hi,
Anyone tried powering this RB5009UG+S+IN via side DC power pins ? We're having problems with this. On power supply which was added to the device everything works fine, POE also works without any problem. But when trying to power this device from side DC braker at our DC power station pops like there is short circuit there. Polarity checked 10 times, everything is ok, Voltage checked 53V so well in the range. It is new device just opened the box.

What am i doing wrong ?
Best regards,
Wojciech Wrona

Maybe your external power has + connected to case and the router has - connected to case?

Maybe your external power has + connected to case and the router has - connected to case?

I'm not sure if in DC circuits any of pins should be connected to case, i think both should be floating cuz cases have connection to rack and racks are grounded everywhere, but i will check it. This is interesting... if any of pins shall be connected to case, (-) is the only one i would think of. Our power station is powering couple of servers, some OLTs and other equipment. Everything works fine. Only this RB5009 is problematic. This is short circuit for sure cuz besides braker pop there is voltage drop at this moment and every equipment connected to this DC line noticed this drop, some of them even rebooted becouse of this.

Best regards,

I think @pe1chl is right: telco DC power supply is nominally -48V, so positive on chasis. IT gear, if DC powered, is almost always +48V, so negative on chasis.

I think @pe1chl is right: telco DC power supply is nominally -48V, so positive on chasis. IT gear, if DC powered, is almost always +48V, so negative on chasis.

Interesting. Didn't know that. Learning all the time. I'll try to confirm this tomorrow, but if it is true are there any ways to overcome this issue besides some DC-DC converters ?

Best regards,
w0jtas

I don't think you can get around this "positive-negative" mismatch without DC-DC converter.

Unless you're willing to mount RB so that metallic parts of its chassis don't touch metallic parts of rack and other devices (i.e. have its chasis galvanically isolated from the rest of your DC). You could put it on a non-conductive shelf (plastic or wooden) but you would still risk shortcuts when somebody would be working around your RB.

Ok, seems cause is confirmed. Thanks everyone. Now couple words about solutions below.

I don't think you can get around this "positive-negative" mismatch without DC-DC converter.

Unless you're willing to mount RB so that metallic parts of its chassis don't touch metallic parts of rack and other devices (i.e. have its chasis galvanically isolated from the rest of your DC). You could put it on a non-conductive shelf (plastic or wooden) but you would still risk shortcuts when somebody would be working around your RB.

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It's possible to use isolated DC-DC converter here or try to reverse polarity on the plug.

Thank You both. It looks lilke isolated DC-DC converter or separate AC-DC PSU would be the best option for me (perhaps second one would be easier to work with). For other ppl googling around about this. Puting the device in isolated environment electricaly does the job but is quite unreliable too. First of all even if You isolate some parts of the case Your're creating 53V potential between the device (mikrotik) and every other metalic part in server room. It's like children's game "not to touch the lava". Every time You touch it can shock You, every time You touch it with something metalic there will be a lot of sparks, braker trips and voltage on DC line drops for a while rebooting every device powered from this line. Not a good idea in proffesional use.

Reversing polarity is a no go too. And probably it WILL NOT work at all, becasue grounded (+) is not the same as reversed polarity. In grounded (+) circuit polarity is still the same, voltage is counted backwards but (+) is still (+) and (-) is still (-), so in normally connected circuit charge is still suppose to flow the same direction. When You reverse polarity You're also reverse charge flow direction which in best way does not work at all, in the worst it will create some smoke in the device which is not protected from "reverse polarity error". Don't do it

Thanks a lot again everyone
Best regards
w0jtas